The invention relates to a process for the metallurgical treatment of pig iron melts in a container, more particularly a transfer ladle, which have an acid oxidized initial slag, by the injection of fine-grained solids into the melt with a carrier gas via an injection lance, and also to a variant wherein the initial slag is removed prior to the treatment of the pig iron melt and a new covering slag is formed.
In the desulphurization of pig iron it is known to fluidize separately two treatment materials, for example, an alkaline earth carbonate, such as calcium carbonate, and a metallic reducing agent, such as fine-grained magnesium (DE OS 27 08 424, DE OS 26 50 113, U.S. Pat. No. 3,998,625) or a mixture of two treatment materials and a further treatment material, for example, calcium carbide and a gas-generating coal in the form of a mixture and fine-grained magnesium (EP OS 0 226 994), or three treatment substances, for example, calcium carbide, a gas-generating coal and finely divided magnesium (U.S. Pat. No. 4,832,739) and to unite the fluidates immediately prior to their entry into the melt.
This injection technique (known as coinjection) has led to an improvement in the desulphurization of pig iron melts. In a further development of the coinjection of calcium carbide and magnesium in a ratio of 3.5:1, a preliminary treatment and an after-treatment have also been performed using approximately 90 kg and approximately 136 kg of calcium carbide respectively (73rd Steel-making Conference Proceedings, Vol. 73, Detroit Meeting, March 25-28, 1990, pages 351-355). Another similar process is known from DE OS 39 42 405. However, disadvantages continue to occur:
1. Pig iron produced in a blast furnace has an acid oxidized initial slag. If, as in the aforedescribed prior art processes, desulphurization is performed in the initial phase with fine-grained magnesium, the magnesium sulphides (MgS) forming may not be absorbed by the slag. PA0 2. It is impossible to adjust an optimum adequate dispersion of the desulphurization agents or a circulatory movement of the bath adapted to the course of the reaction. PA0 3. It is impossible to perform an optimum feed of finely divided magnesium adapted to the sulphur content of the pig iron melt at any particular moment. PA0 4. It is impossible to achieve an optimum conditioning/modification of the final slag - i.e., to reduce the absorption of iron. Furthermore, it is impossible to perform the optimum flushing out of floating magnesium sulphides in the melt or to let dissolved magnesium to react with the sulphur of the melt. PA0 in the initial phase such solids are injected which deoxidize the initial slag and increase its basicity and also produce a circulatory movement of the melt, PA0 in the middle phase one or more desulphurizing agents is or are injected for the main desulphurization, and PA0 in the final phase such solids are injected which purify the melt and produce a final desulphurization and also to influence the desulphurization slag formed that its content of iron granules is low. PA0 in the initial phase (I) deoxidizing and/or lime-containing and/or gas-generating solids are injected, PA0 in the middle phase (II) of the treatment desulphurizing substances in the form of magnesium carriers and/or calcium compounds are injected, possibly with gas-generating solids and fluxing agents.
It is an object of the invention to influence the initial slag so that it can absorb the magnesium sulphides and to ensure an optimum dispersion of the desulphurization agents in the melt with a circulatory movement of the bath which is optimally adapted to the course of the reaction. Another aim is to eliminate interfering influences due to an uncontrolled resulphurization, so that the addition of fine-grained magnesium is better adapted to the sulphur content of the melt, and the modification of the slag to limit the absorption of iron therein and also the flushing of the magnesium sulphides out of the melt can be improved.